CDMA TECHNOLOGY

DEFINITION OF CDMA TECHNOLOGY

A coding scheme, used as a modulation technique, in which multiple channels are independently coded for transmission over a single wideband channel.

BIRTH OF CDMA

During second world war.

First implemented by the English Allies against the German Army.

How English Allies implemented it ???

Main types of CDMA technology:

m Oncd a eOThe CDMAone can be further classified as IS-94A and IS-95A.IS-94A :- Data rate 14.4 kbps.IS-95A :- Data rate 115 kbps. m OOOOcd a Ocdma2000 can be further divided into its variants namely:

1. 1X EV :- Data rate 307 kbps2. 1X EV-DO :- Data rate 2.4 mbps.Both of these variants uses the carrier of 1.25 MHz

 WCDMA:Wideband CDMA that forms the basis of 3G networks, Developed originally by Qualcomm, CDMA is characterized by high capacity and small cell radius, employing spread-spectrum technology and a special coding scheme. WCDMA uses 5 MHz bandwidth.

CDMA PRINCIPLE

Each access to a common channel needs some form of orthogonality.

The spread-spectrum form of multiple access exploits the orthogonality in the code domain and is termed code-division multiple-access (CDMA).Each user in a CDMA network is assigned a unique orthogonal code from the ‘ family of orthogonal code sequences’ (FOCS).

CDMA2000 systems are currently being deployed in the 450 MHz, 800 MHz, 1700 MHz, 1900 MHz and 2100 MHz bands.

HOW THE SIGNAL IS GENERATED IN CDMA ???

There are five steps in CDMA signal generation.analog to digital conversion vocoding encoding and interleaving channelizing the signals conversion of the digital signal to a Radio Frequency (RF) signal

The use of codes is a key part of this process.

A/D conversion

The first step of CDMA signal generation is analog to digital conversion, sometimes called A/D conversion. CDMA uses a technique called Pulse Code Modulation (PCM) to accomplish A/D conversion.

BACK

Voice compression

The second step of CDMA signal generation is voice compression. CDMA uses a device called a vocoder to accomplish voice compression. The term "vocoder" is a contraction of the words "voice" and "code." Vocoders are located at the BSC and in the phone

BACK

Encoding & interleaving

Encoders and interleavers are built into the BTS and the phones.

The purpose of the encoding and interleaving is to build redundancy into the signal so that information lost in transmission can be recovered.

BACK

CHANNELIZATION

The encoded voice data is further encoded to separate it from other encoded voice data. The encoded symbols are then spread over the entire bandwidth of the CDMA channel. This process is called channelization.

The receiver knows the code and uses it to recover the voice data.

BACK

Digital to RF conversion

The BTS combines channelized data from all calls into one signal. It then converts the digital signal to a Radio Frequency (RF) signal for transmission.

D/A conversion

After the CDMA signal is transmitted, the receiver must reverse the signal generation process to recover the voice, as follows: Conversion of RF signal to digital signal Despreading the signal Deinterleaving and decoding Voice decompression Digital to analog voice recovery

TWO KINDS OF CODES:

WALSH CODES:Walsh codes provide a means to uniquely identify each user on the forward link. Walsh codes have a unique mathematical property--they are "orthogonal PN CODES

Pseudorandom Noise (PN) codes uniquely identify users on the reverse link.

A PN code is one that appears to be random, but isn't. The PN codes used in CDMA yield about 4.4 trillion combinations of code. This is a key reason why CDMA is so secure.

WORKING OF CDMA

CDMA works on the spread-spectrum technology.But what actually this technology means ?The spread spectrum increases the bandwidth of the message signal by a factor N, called the processing gainProcessing gain n = Bss / b In CDMA each bit time is subdivided into m short intervals called chips. Each station is assigned a unique m-bit code or chip sequence. To transmit a 1 bit, a station sends its chip sequence. To transmit a 0 bit, it sends the one’s complement of its chip sequence.

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APPLICATIONS OF CDMA TECHNOLOGY.

Daily Downloads Real time stock quotes Text Communication Sending photos over the air Position Location Services Games and Entertainment

ADVANTAGES OF CDMA

No SIM card is required ,Improved call quality Enhanced privacy ,Increased talk time .Increased data rate.resistant to jammingIncreased Capacity, can handle more customers better and more consistent sound ignore sounds having no useful content

Disadvantages Of CDMA

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Reference

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